This invention pertains to thermally conductive materials, and more particularly, to a thermally conductive interface material for heat generating devices, such as microprocessor power supply assemblies, that facilitates heat transfer from the heat generating device to a heat sink.
With increasing market pressure for smaller, faster, and more sophisticated end products using integrated circuits, the electronics industry has responded by developing integrated circuits which occupy less volume, yet operate at high current densities. Power supply assemblies for such microprocessors generate considerable heat during operation. If the heat is not adequately removed, the increased temperatures generated by the power supply assemblies will result in damage to the semiconductor components.
A heat sink is commonly used to transfer the heat away from the power supply or other heat generating assembly. The heat sink generally includes a plate or body formed from a conductive metal, which is maintained in thermal contact with the assembly for dissipating heat in an efficient manner. Fins optionally protrude from the plate for providing an increased surface area for heat dissipation to the surrounding environment.
The current industry technique for providing thermal contact between a microprocessor power supply assembly and a heat sink is to interpose a thermal interface material between the two, which facilitates heat transfer from the active device to the heat sink.
One method is to apply a ceramic filled thermal grease, which is typically silicone based, between the heat sink and the power supply. Thermal greases provide excellent thermal properties, but require an extensive assembly process with high manufacturing cost. The product is usually applied by hand, from a syringe, or with an aluminum carrier. This process is labor intensive and slow and does not lend itself to automation.
Another method for providing a conductive interface includes the use of thermally conductive wax compounds. These compounds, however, are generally brittle at ambient temperatures and easily chipped off, resulting in a high thermal resistance. The low viscosity of the wax at operating temperature causes the wax to flow out from between the active component and the heat sink, resulting in a high thermal resistance. Further, because of the brittle nature of the wax compounds, they are difficult to manufacture and apply to a heat sink.
Thermally conductive silicone rubbers have also been used as conductive interfaces. Although soft and pliable, the silicone rubber requires relatively high pressure and a long warm-up time to attain a low thermal resistance. The rubbers have poor flow characteristics which result in a low thermal conduction when there is a mismatch of flatness between the heat sink and the heat producing device. Differences in the thermal coefficient of expansion between the silicone rubber and the heat sink can result in high thermal resistance during temperature cycling. These effects lead to a poor thermal conductivity from the heat producing device to the heat sink.
Other thermal interfaces employ polymeric thermally conductive cure-in-place compounds. These compounds are generally rigid after cure. They have a poor reliability because of a difference in thermal coefficient of expansion between the material and the heat sink, causing cracks and failure during temperature cycling. The polymeric materials are labor intensive to apply and require long cure times.
The present invention provides for a new and improved thermal interface which overcomes the above-referenced problems and others.
The present invention relates to a thermal interface material which can be easily pre-attached to a microprocessor power assembly or a heat sink prior to shipment.
In accordance with one aspect of the present invention, a thermal interface material, which undergoes a phase change at microprocessor operating temperatures to transfer heat generated by a heat source to a heat sink, is provided. The thermal interface material includes a phase change substance, which softens at about the operating temperature of the heat source. The phase change substance includes a polymer component and a melting point component. The melting point component modifies the temperature at which the phase change substance softens. The thermal interface material further includes a thermally conductive filler dispersed within the phase change substance.
In accordance with another aspect of the present invention, a multi-layer strip is provided. The strip includes a layer of a thermal interface material for thermally connecting a heat source with a heat sink. The thermal interface material includes a polymer component, a melting point component in sufficient quantity to adjust the softening temperature of the interface material to about the operating temperature of the heat source, and a thermally conductive filler mixed with the melting point component and the polymer component. The strip further includes an outer layer disposed on a side of the thermal interface material. The outer layer includes at least one of a protective releasable liner and a layer of an adhesive material.
In accordance with another aspect of the present invention, a method of providing a thermal interface between a heat source and a heat sink is provided. The method includes interposing a thermal interface material between the heat source and heat sink, which softens at about the operating temperature of the heat source to provide a thermal interface between the heat source and the heat sink during operation of the heat source. The thermal interface material includes a polymer component, a melting component for modifying the temperature at which the thermal interface material softens, and a thermally conductive filler mixed with the polymer component and the melting point component.
One advantage of the present invention is that the thermal interface material can be pre-attached to a heat sink prior to shipment.
Another advantage of the present invention is that the cohesive strength and integrity of the thermal interface material provide for easy handling.
Still another advantage of the present invention is that the thermal performance of the thermal interface material matches that of thermal grease in a solid film form.
Still another advantage of the present invention is that a phase change or softening at the operating temperatures maximizes interfacial surface wetting.
Still another advantage of the present invention is that low application pressure without added heat allows for hand mounting during field rework and processor upgrades.
Still another advantage of the present invention is that the assembly process associated with thermal grease is eliminated but an equivalent thermal performance is maintained.
Still another advantage of the present invention system assembly cost is minimized by allowing for pre-attachment to a heat sink or CPU.
Still another advantage of the present invention is that the material softens and conforms to surface roughness or concavity at operating temperature.
Still another advantage of the present invention is that the material operates at low clip pressures (5 to 10 psi).
Still another advantage of the present invention is that the material can be applied and repositioned with thumb pressure allowing for easy field service.
Still another advantage of the present invention is that the material allows for vertical mounting due to its cohesive properties.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.